Carbon fibers have been used in a wide variety of structural applications and industries because of their desirable properties. For example, carbon fibers can be formed into a structural component that combines high strength and high stiffness, while having a weight that is significantly lighter than a metal component of equivalent properties. Carbon fibers can be manufactured by converting a precursor fiber, such as a spun polyacrylonitrile (PAN) fiber, in a multi-step process in which the precursor fiber is heated, oxidized, and carbonized to produce a fiber that is 90% or greater carbon. The resulting carbon fibers can be molded into high strength composite materials for structural applications, used in their pure form for electrical and friction applications, or can be further processed for use in adsorbent, filter, or other applications. In particular, composite materials have been developed in which carbon fibers serve as a reinforcing material in a resin, ceramic, or metal matrix.
Increasingly, carbon fibers are being used as structural components in aerospace applications. In order to meet the rigorous demands of the aerospace industry, it is necessary to continually develop new carbon fibers having both high tensile strength and high modulus of elasticity. In particular, there is a desire to develop carbon fibers having a tensile strength of 1,000 ksi or greater and a modulus of elasticity of 50 Msi or greater. Carbon fibers having individually higher tensile strength and modulus can be used in fewer quantities than lower strength carbon fibers and still achieve the same total strength for a given carbon fiber composite component. As a result, the composite component weighs less. A decrease in component weight is important to the aerospace industry and increases the fuel efficiency of aircraft incorporating such a component.
Several methods of increasing tensile strength and modulus have been explored in the prior art, and have generally had mixed results. For example, it is generally known that modulus can be increased by increasing carbonization temperatures. However, increases in carbonization temperatures result in a decrease in tensile strength. As a result, this method has generally not provided an effective means for preparing carbon fibers having improved tensile strength and modulus of elasticity.
Other methods have focused on stretching the precursor fibers before or during the process of converting the precursor fiber to a carbon fiber. It has previously been recognized in the prior art that the modulus of carbon fibers can be improved by stretching the fibers in a post-spinning step, oxidizing step, carbonizing step, or a combination thereof. However, conventional wisdom believed that the amount of stretching in the oxidizing step was limited by tension levels in the fibers that developed in response to the onset of chemical reactions, such as thermally induced cyclization and/or oxidative crosslinking of the PAN precursor fibers. The accumulation of tension caused the fibers to break at relatively low stretches under standard oxidation conditions, e.g., above 180° C. As a result, prior attempts to stretch PAN fibers during oxidation have generally been limited to a maximum amount of stretch or to a single continuous stretch.
Several studies and prior art references have further indicated that improvements beyond this initial or maximum stretch provide little if any gain in properties, and in fact may actually lead to breakage or damage in the fibers. For example, U.S. Pat. No. 4,609,540 describes a method of determining the optimum stretch to be applied to a precursor fiber in an oxidizing atmosphere. According to the '540 patent, the optimum amount of stretch corresponds to an inflection point that is determinable from a plot of % elongation versus tension, and that this optimum elongation also roughly corresponds to the maximum degree of crystalline orientation within the fibers. Beyond this inflection point, the '540 patent teaches that any gains from further stretching are minimal and may result in the development of fluff and possibly breakage.
Thus, there exists a need for carbon fibers having both high tensile strength and high modulus of elasticity, and for a method and apparatus that can be used to prepare such carbon fibers.